1. Field of the Invention
This invention relates generally to the field of analog integrated circuit design and, more particularly, to oscillator and timer design.
2. Description of the Related Art
Oscillators play a prominent role in the functionality of a large portion of today's analog and digital systems. Typically, oscillators, also referred to as astable multivibrators, are electronic circuits that convert energy from direct-current sources into periodically varying electrical signals, or voltages. In other words, an oscillator typically operates by utilizing the electrical behavior of its circuit elements to convert a steady state input signal into a periodic, time variant output signal. In some implementations the signal produced by an oscillator may be sinusoidal in appearance, such as a sine wave, in other implementations it may appear as a square wave, triangular wave, or a variety of other repeatable signals. Many of today's integrated circuits that require oscillators, such as timer circuits and Phase-Locked Loops (PLLs), need to include the oscillators on-chip in order to meet cost and area requirements. The behavior of such on-chip oscillators is typically affected by the technology used to fabricate the integrated circuit. For example, many widely used fabrication processes today are based on complementary metal-oxide-semiconductor (CMOS) technology, where each specific qualified CMOS process varies slightly from another.
One common type of oscillator is the relaxation oscillator. Typically a relaxation oscillator achieves its oscillating output by charging a capacitor to some event or switching threshold. The event discharges the capacitor, and its recharge time determines the repetition time of the events or switching. Similarly, an oscillating output could also be achieved by discharging instead of charging the capacitor to reach the event or switching threshold. Typically the capacitor is charged through a resistor, where the values of the resistor and the capacitor, referred to as the RC time constant, determine the rate, or frequency, of the oscillation. For example, decreasing the value of the resistor may increase the oscillation frequency, and increasing the value of the resistor may decrease the oscillation frequency. In the case of a typical relaxation oscillator whose frequency is determined by an RC time constant, any fabrication process variation on any of the parameters (namely R and C) will typically result in a shift in the oscillator frequency.
One widely used oscillator topology in the art is found in the popular IC555 timer circuit, which has been on the market since the mid-Nineteen Sixties. The IC555 timer is frequently configured in a free-running mode, where a capacitor between an upper threshold, which is determined by a first comparator, and a lower threshold, which is determined by a second comparator, is resistively charged and discharged. The frequency of oscillation of the timer may be affected by various factors, including finite comparator gain and offset voltage, temperature variations, and process shift in silicon processing.
The IC555 has been widely used in the art, and methods have been developed to minimize changes in the frequency of oscillation resulting from temperature effects. Efforts have also been made to counteract the effects of finite comparator gains and offset voltages, by making the frequency of oscillation a value independent of the supply voltage used by the respective circuits. Typically, these solutions employ discrete precision resistor and capacitor components that are external to the oscillator circuit in order to avoid the problems caused by process shift. As a result, these solutions are generally not ideal for systems or circuits that require an integrated oscillator with reduced pin counts. When working within generally tight operational tolerances, a trim capability of the circuit may be required to make the necessary adjustments needed for achieving proper circuit operation over variations present in silicon processing.
Trimming is typically accomplished by cutting (or blowing) fuses, which results in a permanent change. The outputs of the fuses are generally a couple of digital bits, also called trim bits. The trim bits are sometimes implemented as a programmable option, where during power-up they are latched to pre-specified levels. For greater flexibility the trimming is many times performed using a combination of both methods. Typically, when performing trimming for an oscillator, the digital bits select different RC time constants to set the correct output frequency regardless of any process shift of the internal integrated components. Generally a bank of capacitors and/or a bank of resistors are employed for selecting the appropriate RC time constant. If the effects of the process shift are diminished, the trim range can be effectively decreased leading to smaller area requirements for the capacitor bank and/or resistor banks.
Therefore, there exists a need for a system and method for designing an integrated relaxation oscillator that exhibits minimal change in the frequency of oscillation caused by process variation, by improving sensitivity to component variation due to process shift while minimizing the area requirements for capacitor banks and/or resistor banks used during trimming.